1. Field of the Invention
This invention relates in general to a chuck used to hold a substrate during semiconductor processing, and more particularly to a system and method for controlling and maintaining uniform temperature of the substrate by adjusting pressure of the cooling gas maintained between the chuck and the substrate.
2. Description of the Related Art
In processes involving the fabrication of semiconductor integrated circuits, electronic components, and devices, various support systems have been employed to support the substrate, such as a silicon wafer, during the processing of the wafer. In certain processes such as lithography, image quality considerations require that the wafer be held level by a chuck in such a way that its exposed surface is flat, with little distortion, which can cause image misalignment.
One type of support system employs a perimeter-clamping ring which extends along the periphery of the wafer to retain the wafer in place. The portion of the wafer beneath the ring is clamped tightly against a support member. In such a system, the clamping ring reduces the total available area for circuit formation since the peripheral edge of the wafer is generally covered by the ring. Further, distortion to the wafer surface may result from particles trapped between the wafer and the support member.
Another type of support system employs a vacuum to hold the wafer against a chuck. In the vacuum support system, the wafer is held against the chuck by the use of a vacuum pump, which reduces the interior pressure in the space between the chuck and the wafer to a point that is lower than the exterior gas pressure of the chamber environment, in which the support system operates. Many semiconductor production processes, such as plasma etching, are performed in a high vacuum chamber environment (10xe2x88x923 Torr or less). The vacuum support system typically cannot operate well in such high vacuum chamber environment because the vacuum support system requires that the chamber environment has a higher pressure than the interior of the support system in order to hold the wafer down against the chuck.
Another type of support system uses electrostatic forces to clamp a wafer to a support surface. Typically, in such a system, substantially all or all of the surface area of the wafer may become available for processing and the chuck can effectively be used in a high vacuum exterior environment. A typical electrostatic chuck includes a base having a surface on which the wafer is supported and an electrostatic member that is electrostatically biased with respect to the wafer by an electrical potential. The wafer is held in place against the chuck""s support surface by electrostatic forces.
During wafer processing, heat is often generated, and it is often necessary to limit the maximum temperature rise of the wafer. It is also important to maintain temperature uniformity over the wafer surface. If there are excessive temperature variations across the wafer surface (often due to poor and/or non-uniform heat transfer), the wafer can become distorted. In a vacuum environment, the transfer of heat out of the wafer is less efficient, because the heat transfer is accomplished mainly by radiation. The support system often provides a way of cooling the wafer in an attempt to maintain substantially constant wafer temperature. If the contact surface between the chuck and the wafer is smooth, the contact area is sufficient, and the thermal conductivity of the chuck is high, a substantial amount of heat can be transported from the wafer to the chuck and out through a heat exchanger. If the contact surface of the wafer or chuck is rough or the contact area is limited, the heat transfer via the chuck can be substantially reduced. Achieving intimate contact between chuck and wafer is difficult. Furthermore, this condition is normally avoided because of the risk of particles being trapped between the chuck and wafer, possibly leading to wafer distortion. In this situation an inert gas such as Helium (He) can be used as a thermal conductor to fill the space between the wafer and the chuck in order to conduct heat out of the wafer via the underside of the wafer. A fluid could also be flowed between the wafer and the chuck substrate, to convectively cool the wafer. However, maintaining sufficient clamping force, and avoiding serious leaks to the surrounding vacuum, would be difficult.
U.S. Pat. No. 4,565,601 describes an electrostatic chuck and a method of controlling the temperature of a wafer being processed in a vacuum at a predetermined temperature. The electrostatic chuck clamps and supports the wafer both at its periphery and at interior points. A gas coolant is supplied to the gap between the chuck top and wafer bottom. The clamping region at the periphery provides a seal to prevent the flow of gas into the vacuum chamber. However, if the peripheral contact region does not make good contact with the back of the wafer, for example, due to particles trapped in between or a non-flat surface of the wafer, leakage into the chamber will occur. Also, according to the patent, it requires clamping of the chuck and the wafer at the periphery to provide a seal to prevent the flow of gas coolant into the ambient vacuum. In order for the seal to be effective, a significant amount of clamping pressure must be applied, which can potentially distort the wafer.
Electrogrip Inc., manufactures a Helium-cooled electrostatic chuck with a leak rate of 0.5-1.0 sccm, for 200 mm wafers and a gas pressure of 10 Torr. This is equivalent to a throughput of about 0.013 Torr-l/sec at atmospheric pressure, 760 Torr. In order to maintain a chamber pressure of 1 mTorr, typical of etching systems, with this leak rate a difficult to achieve such pumping speeds. However, for other applications, such as electron beam lithography, chamber pressures of 10xe2x88x926 to 10xe2x88x927 Torr or better are required. Maintaining these vacuum levels with such a chuck leak rate would require pump speeds of 103 to 104 times larger, which is no longer feasible.
A need exists for an electrostatic chuck that provides uniform transfer of heat over the entire working area of the wafer, minimizes leakage of cooling gas into the operating environment, and secures the wafer without distortion.
The present invention overcomes the drawbacks of the prior art by adding an annular array of gas inlets toward the periphery of an electrostatic chuck, to provide a uniform distribution of cooling gas to the held substrate, and a gas bearing seal at the wafer periphery. In one particular embodiment, the present invention provides a uniform distribution of gas to a wafer held by an electrostatic pin chuck that has an annular array of gas inlets located within an annular array of gas outlets at the periphery of the chuck. Gas entering from the array of gas inlets helps to equalize the cooling gas pressure in the chuck cavity. The uniform distribution of gas results in a uniform transfer of heat from the wafer. Two annular rims surrounding the annular array of gas outlets towards the outer perimeter of the chuck help to prevent leakages of gas into the vacuum by functioning as gas bearing seals.
The gas is supplied from a reservoir of sufficient size that essentially constant gas pressure between wafer and substrate is maintained, despite the loss of gas at the gas outlets.